In many industrial processes using a gaseous feed stream it is desirable or necessary to remove carbon dioxide from the gaseous feed stream prior to certain steps of the processes. For example, in the separation of atmospheric air into its component parts by cryogenic distillation, it is necessary to prepurify the air by removing carbon dioxide and water vapor therefrom prior to refrigerating the air; otherwise, these impurities would condense and freeze in the refrigeration heat exchange equipment and eventually clog the equipment, thereby necessitating removal of the equipment from service for removal of the frozen carbon dioxide and ice. The carbon dioxide and water vapor can be removed from the air by a number of techniques, including the use of pairs of reversing heat exchangers that are operated alternately, such that one heat exchanger is in purification service while the other is undergoing frozen carbon dioxide and ice removal, and pressure swing adsorption (PSA) and temperature swing adsorption (TSA) processes.
A very effective PSA method for removing carbon dioxide and water vapor from gas streams involves the use of activated alumina as the sole adsorbent for removing both carbon dioxide and water vapor from gas streams. This process is disclosed in U.S. Pat. No. 5,232,474, the disclosure of which is incorporated herein by reference. Although this method is superior to other prior art methods of prepurifying air, the activated alumina adsorbent is not very effective for reducing the concentration of carbon dioxide in air down to very low levels, for example &lt;&lt;1 ppm, when the activated alumina adsorbent is contaminated by even a very low level of water vapor. Therefore, a large amount of activated alumina adsorbent is required to produce high purity air, resulting in a substantial increase in the cost of the PSA air purification process. Furthermore, the PSA air purification process is very vulnerable to upset operation when excessive water vapor and/or liquid water are introduced into the activated alumina-containing vessels. Once the PSA process is upset, the adsorbent performance will deteriorate, and carbon dioxide may break through into the purified gas stream. Under normal PSA operating conditions it may take as long as one year for the bed to restore itself to satisfactory performance.
It would be desirable to improve the efficiency of activated alumina-based PSA processes for the removal of low levels of carbon dioxide from gas streams. The present invention provides such an improvement.